Telephony encompasses the general use of equipment to provide voice communication over a distance. Plain old telephone service, or POTS, is the most basic form of residential and small business telephone service. POTS became available soon after the telephone system was introduced in the late 19th century and, from the standpoint of the user, has existed more or less unchanged ever since. POTS services include bi-directional or full duplex voice path with a limited frequency range, a dial tone and ringing signal, subscriber dialing, operator services such as directory assistance, and long distance and conference calling. During the 1970s and 1980s, new network services became available due to the creation of electronic telephone exchanges and computerization. New services included voice mail, caller ID, call waiting, reminder calls and other similar services.
Advances in digital electronics have revolutionized telephony by providing alternate means of voice communication than those provided by traditional (analog) telephone systems. IP Telephony is a form of telephony which uses the TCP/IP protocol popularized by the Internet to transmit digitized voice data. The routing of voice conversations over the Internet or through other IP networks is also called VoIP (Voice over Internet Protocol). Digital telephony was introduced to improve voice services, but was subsequently found to be very useful in the creation of new network services because it can transfer data quickly over telephone lines. Computer Telephony Integration (CTI) enables a computer to control phone functions such as making and receiving voice, fax, and data calls. The Session Initiation Protocol (SIP) is a signaling protocol used for creating, modifying and terminating sessions (voice or video calls) with one or more participants. Sessions include Internet telephone calls, multimedia distribution and multimedia conferences. Development of the SIP protocol was motivated by the need for a signaling and call setup protocol for IP-based communications that could support the call processing functions and features present in the public switched telephone network (PSTN) using proxy servers and user agents.
All the advances in telephony, however, have not solved some very basic problems associated with actually reaching a desired party. It is often difficult to reach people by telephone. The caller dials a series of numbers or otherwise selects a number with which he seeks connection and hopes that the call will be connected to the callee's telephone, and that the callee will answer the call. If the call is not connected because the telephone line is busy or because the caller has misdialed the number, the caller will typically have to call back or leave a voice mail message in the hope that the callee will eventually listen to the message. Making frequent calls to the same service (e.g., long distance or calling card services, information services like banks) typically requires repetitive input of information (e.g., PINs Personal Identification Numbers) that is prone to being incorrectly input or to being forgotten. Furthermore, when a caller is placing a call, he or she typically has no way to conduct other telephony communications other than those provided at the line level (e.g., hold/transfer/conference functions).